It's a complex problem that hasn't even been formulated properly yet. For example, every existing solution touts "security" and yet completely mangles the diff

It's a complex problem that hasn't even been formulated properly yet. For example, every existing solution touts "security" and yet completely mangles the difference between authentication and encryption. Authentication is important - you don't want random servers or users to enroll on your network, and you want good tools to rotate and manage secrets. Encryption isn't important unless you care about state-level actors sniffing your traffic at the backbone. (And if you care about that then you already have your own datacenter.) Meanwhile encrypting all network traffic is a huge performance penalty. (Orders of magnitude for some valid use cases.)